The present invention relates to a laminated lead frame used for a semiconductor device, such as an IC, and more particularly to a laminated lead frame formed by laminating a plurality of thin plates and to a method for manufacturing the same.
In the field of QFN (Quad Flat Non-leaded) semiconductor devices, SON (Small Outline Non-leaded) semiconductor devices, and the like, a reduction in the pitch of leads (inner leads) has been pursued with an aim of achieving a reduction in thickness. There is a trend toward a further reduction in the thickness of a lead frame. However, when the thickness of the lead frame is reduced, the lead frame is prone to buckling during a transport process or a subsequent assembly process, to thus induce a failure, which is in turn responsible for a reduction in productivity.
In a related-art QFN semiconductor device or a related-art SON semiconductor device, only external contact terminals are exposed outside a sealing resin section of a package, and other areas of the device are covered with sealing resin. Eventually, an increase arises in the thickness of resin in only this area, thereby hindering a reduction in the thickness of a package. For this reason, there has been known a laminated lead frame, wherein two lead frames of different shapes are laminated in an attempt to reduce the size and thickness of an IC package while the strength of the lead frame is maintained, thereby forming a single laminated lead frame (see e.g., Patent Document 1). In addition, a package (see, e.g., Patent Document 2) using a lead frame—whose thickness corresponds to the thickness of a package and which has an L-shaped cross-sectional profile—has hitherto been known.
These laminated lead frames are created by a technique of bonding together a plurality of lead frame single plates (referring to individual lead frame materials to be bonded), thereby rendering a pitch of a lead frame fine and enabling materialization of a lead frame of three-dimensional profile. A diffusion bonding method is primarily used for bonding lead frame single plates together, wherein an appropriate load and heat are applied to the laminated lead frame single plates from a thickness wise direction thereof.
Under the diffusion bonding method, as a result of application of a load to materials (lead frame single plates) remaining in a laminated state, mutually-facing interfaces of the materials move closer to each other. When the materials are heated in this state, atomic energy in the materials becomes active, thereby initiating migration (diffusion) of atoms between the materials adjoining to each other with the interfaces interposed therebetween. When diffusion of atoms proceeds, atoms of the two materials are mingled together to such an extent that the presence of the interfaces becomes uncertain, whereby the materials are bonded together.
According to the diffusion bonding method, approach (deformation) of the interfaces achieved by application of a load generally becomes more likely to arise at a higher temperature, thereby facilitating a progress in diffusion and enhancing a bonding characteristic.
However, when the diffusion bonding method is adopted in manufacture of a laminated lead frame, diffusion bonding is realized by means of applying a heavy load of an order which limits plastic deformation (a reduction in thickness) of a lead frame single plate to one percent or less (namely, when bonding load is increased, waviness or irregularities existing in the interface are mechanically squeezed, thereby enabling a reduction in a distance between atoms of upper and lower lead frame single plates), because diffusion bonding is desired to be performed at as low a temperature as possible (a temperature used for assembling an IC; e.g., 260° C. or less).
The surface of a lead frame single plate is plated as an inserted material to a thickness of the order of 3 to 5 micrometers with Ag or Au which has a low fusing point and exhibits high diffusion with respect to a material (Cu or Fe—Ni) of the lead frame single plate. As a result, an attempt is made to reduce the bonding temperature and load.
Moreover, there has been proposed a laminated lead frame formed by laminating a lower lead frame and an upper lead frame (see, e.g., Patent Document 3). By means of the laminated lead frame, the number of outer leads is increased, and an increase in the width of a plastic sealing body even in the semiconductor device can be avoided to the extent possible.
Moreover, a semiconductor device using a laminated lead frame is also proposed (see, e.g., Patent Document 4).
[Patent Document 1] JP-UM-B-7-13227
[Patent Document 2] JP-A-2003-7955
[Patent Document 3] JP-A-2001-035987
[Patent Document 4] JP-A-2001-274310
According to the technique described in Patent Document 1, in order to achieve success in bonding together lead frame single plates at the lowest possible temperature and under the lowest possible load, surface roughness and flatness of planes of both lead frame single plates to be bonded together must have been increased so that the two materials can make contact with each other in an early stage.
For instance, as shown in FIG. 14, when a terminal lead 60 of an upper lead frame single plate and an inner lead 61 of a lower lead frame single plate must be bonded together, the entire leads 60 and 61 are, at present, brought into contact with each other and bonded together. According to this method, an area where the leads 60 and 61 are bonded together is wide, and hence difficulty is encountered in maintaining the flatness of the surfaces of the leads 60 and 61. In FIG. 14, reference numeral 62 denotes a bond portion.
Particularly, variations in the thickness of the Ag-plating or the thickness of the Au-plating, both elements being used as an insert material, range from about 1 to 2 micrometers, and there is a characteristic of the element being deposited to a large thickness at the end of the lead. Hence, as shown in FIG. 15, when lead frame single plates are stacked together, a clearance 63 arises in the center of the leads 60, 61. Accordingly, when the leads are bonded together in this state, an unbonded portion remains, which poses difficulty in assuring perfect bonding over the entire surface. In order to bring the interfaces into contact with each other by collapse of the clearance and to achieve success in bonding the interfaces, a much heavier load is required. Consequently, plastic deformation (a decreased thickness) leads to occurrence of a failure in the shape of the lead frame, and a large press machine used for generating a heavy load is required.
Under the method described in Patent Document 3, when a single laminated lead frame is formed by overlapping, one on top of the other, at least two lead frame single plates, which have been processed into a predetermined shape in advance (corresponding to single lead frame materials before being superimposed or stacked together), registration of upper and lower lead frame single plates is considerably laborious. Pilot holes are generally formed in both sides of a bar used for manufacturing a lead frame single plate.
In relation to bonding of the upper and lower lead frame single plates, the upper and lower lead frame single plates are placed in a bonding mold, and the single plates are positioned by means of formed pilot holes and pilot pins. After the upper and lower lead frame single plates have been positioned, the plates are heated and pressurized within the bonding mold, thereby completing a laminated lead frame into which the upper and lower lead frame single plates are bonded together.
As mentioned above, the bonding mold requires pilot pins used for positioning each of the lead frame single plates, and a plurality of bonding molds must be prepared according to the type of a lead frame single plate. During the course of manufacture of a lead frame single plate, pilot holes acquired after bonding are narrowed by minute misregistration, or the like, of the lead frame single plates, and galling arises in the pins, which in turn poses difficulty in removal of a laminated lead frame. This is responsible for deformation of a laminated lead frame or defects.
Occasionally, there also arises a case where specifications provided by the client pose difficulty in forming pilot holes at required positions on side rails of a laminated lead frame because of restrictions, such as difficulty in transporting a laminated lead frame by use of, e.g., a suction pad.
Moreover, when the lead frame single plates and a completed laminated lead frame are subjected to registration by use of the pilot holes, accuracy is required in connection with the positions of the pilot pins, a pitch between pilot pins of adjacent lead frames (or unit lead frames), the outer shape of a lead frame single plate, and the like, which leads to an increase in required quality or accuracy of jigs, and the like, and an accompanying cost hike.